The present invention relates generally to methods of laser processing and modification of materials, and more particularly the present invention relates to laser processing and modification of a variety of materials using ultrafast laser pulses.
Many efforts in the current generation of laser processing of materials can be described as investigating new modalities in which the laser fluence may be delivered to a workpiece, specifically the ways in which the pulse duration, wavelength or pulse-shape give significant new control over the laser-material interaction.
Various studies have shown that laser material processing in the ultrashort-pulse regime ( less than 100 picosecond) offers numerous advantages compared with longer pulses, see for example S A. Kuper and M. Stuke, Appl. Phys. B 44, 2045 (1987); S. Press and M. Stuke, Appl. Phys. Lett 67, 338 (1995); C. Momma et al., Optics Comm., 129, 134 (1996); C. Momma et al., Appl. Surf. Sci., 109/110, 15 (1997); D. von der Linde, K. Sokolowski-Tinten, and J. Bialkowski, Appl. Surf. Sci. 109/110, 1 (1997); X. Liu, D. Du, and G. Mourou, IEEE J. of Quantum Electron. 33, 1706 (1997) J. X. Zhao, B. Hxc3xcttner, and A. Menschig, SPIE Proc Vol. 3618, (1999); U.S. Pat. No. 5,361,275; U.S. Pat. No. 5,656,186; U.S. Pat. No. 5,720,894; U.S. Pat. No. 6,090,507; U.S. Pat. No. 6,150,630; U.S. Pat. No. 6,043,452; and patent publication WO 89/08529. The first reported advantages in ultrafast laser processing by S A. Kuper and M. Stuke, Appl. Phys. B 44, 2045 (1987) and patent publication WO 89/08529 emphasized improvements in surface morphology, absence of thermal degradation, and reduced threshold fluence for polymers and inorganic non-metallics such as teeth when using sub-picosecond ultraviolet lasers in comparison with traditional nanosecond ultraviolet lasers. Ultrashort lasers offer high intensity to micromachine, to modify and to process surfaces cleanly by aggressively driving multi-photon, tunnel ionization, and electron-avalanche processes, see J. Ihlemann, Appl. Surf. Sci. 54 (1992) 193; D. Du, X. Liu, G. Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64 (1994) 3071; P. P. Pronko, S. K. Dutta, J. Squier, J. V. Rudd, D. Du, G. Mourou, Optics Comm. 114 (1995) 106; B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchick, B. W. Shore, M. D Perry, J. Opt. Soc. Am B 13 (1996) 459; and C. B. Schaffer, A. Brodeur, N. Nishimura, and E. Mazur, SPIE 3616 (1999) 143.
Beyond the simple delivery of xe2x80x98rawxe2x80x99 fluence, lasers offer the parameters of intensity, wavelength, and pulse duration as factors which afford control over essential aspects of material interaction. Particularly, ultrafast laser interactions have well-defined xe2x80x98damagexe2x80x99 thresholds offering improved precision in processing applications, including the fabrication of hole sizes that are smaller than the beam diameter, see U.S. Pat. No. 5,656,186; X. Liu, D. Du, and G. Mourou, IEEE J. of Quantum Electron. 33, 1706 (1997) and D. Du, X. Liu, G. Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64 3071 (1994). Much recent literature has been devoted to ultrafast laser damage and processing of transparent or wide-bandgap materials, see J. Ihlemann, Appl. Surf. Sci. 54 (1992) 193, D. Du, X. Liu, G. Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64 (1994) 3071. Nonlinear absorption mechanisms are key to coupling laser energy into such non-absorbing media.
The thermal impact of picosecond and femtosecond laser interactions is highly limited, confining laser energy dissipation to small optical penetration depths with minimal collateral damage. This precisely confined laser xe2x80x98heatingxe2x80x99 minimizes the energy loss into the underlying bulk material, providing for an efficient and controllable ablation process, see U.S. Pat. No. 5,656,186; U.S. Pat. No. 5,720,894; U.S. Pat. No. 6,150,630; S. Preuss, A. Demchuk, and M. Stuke, Appl. Phys. A, 61, 33 (1995); and T. Gxc3x6tz and M. Stuke, Appl. Phys. A, 64, 539 (1997). Because the laser-matter interaction is so brief, there is a shift in the partition of absorbed energy. Relatively thin layers of near-solid density material are heated, during ultrafast-laser interaction, and this enhances evaporative cooling: though the speed of expansion of the volume of heated material is largely fixed by the temperature, the factor increase in volume of a thin layer is much greater. The volume of tenuous heated material more quickly decouples thermally from the bulk, in the case of ultrafast laser-matter interaction, and in this brief time less heat is transferred from the laser-absorption zone to the underlying bulk material. A greater proportion of absorbed energy is carried away in the evaporated material than is the case for longer-duration pulses.
Collectively, these ultrafast laser effects in small volumes minimize thermal transport, mechanical shocks, cracks, charring, discolouration, and surface melting in the nearby laser interaction zone. Ultrafast laser machining permits repair of ultrafine (sub-mircron) defects on photomasks, see U.S. Pat. No. 6,090,507. Such interactions also reduce pain during medical procedures (see U.S. Pat. No. 5,720,894) and enable the microshaping of explosive materials without deflagration or detonation (see U.S. Pat. No. 6,150,630). The short duration further ensures that, all of the laser energy arrives at the surface before the development of a significant ablation plume and/or plasma; such efficient energy coupling is not available with longer duration ( greater than 10""s ps) laser pulses because of plasma reflection, plasma and plume scattering, and plume heating. Such ultrafast-processing features are highly attractive for the precise microprocessing of good heat conductors such as metals; at the same time, nonlinear absorption of these intense ultrafast pulses also reduces the ablation threshold for wide-bandgap or xe2x80x9ctransparentxe2x80x9d optical materials such as silica glasses.
Ultrafast lasers also offer the means to internally process transparent glass. Microexplosions provide opportunities for 3-D optical storage (C. B. Schaffer, A. Brodeur, N. Nishimura, and E. Mazur, SPIE 3616(1999) 143) while refractive index structures such as volume gratings and waveguides (K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21 (1996)1729) have been formed, by the permanent alteration of the local index of refraction.
These prior studies and developments of ultrashort-laser processing of materials have centered on ultrafast systems with pulse rates typically operating in the xcx9c1 Hz to 10,000 kHz regime. A high-repetition rate three-pulse laser system is described by Opower in U.S. Pat. No. 5,361,275 with pulse separations of 0.5 to 5 ns (200 to 2000 MHz); each pulse is a different wavelength, delivered such that a subsequent pulse arrives soon enough to still interact with the expanding plume of the previous pulse, thereby to benefit from more uniform heating of the plasma plume.
While ultrafast lasers offer exciting prospects for processing materials, at present undesirable effects exist and processing windows are poorly defined. Effects requiring more control in laser processing and modification of materials includes, for example, incubation (defect generation) effects that change etching rates, self-focusing and clouding effects, xe2x80x98gentlexe2x80x99 and xe2x80x98strongxe2x80x99 ablation phases developing with increasing number of pulses, pre-pulse or pedestal effects, poor morphology,: periodic surface structures, melt, debris, surface swelling, shock-induced microcracking, slow processing rates and saturation of hole depth in via/hole formation.
It is advantageous to provide a method of laser processing of materials that addresses the aforementioned difficulties present in present processing methods.
The present invention provides a method of processing and/or modifying materials based on high repetition-rate (continuous or pulsetrain-burst) application of ultrafast laser pulses to materials. The high-repetition rate provides a new control over laser interactions by defining the arrival time of subsequent laser pulse(s), for example: to be after the timescale of plasma-plume expansion and dissipation, but before thermal and other relaxation processes in the material have fully evolved. In one embodiment, the present invention provides a novel method of controlling the delivery of laser fluence to a material during laser processing that reduces unwanted damage in the material.
In one aspect of the invention there is provided a method of laser induced modification of a material, comprising:
applying at least one burst of laser pulses to a material, the laser pulses having a time separation between individual laser pulses in a range appropriate so as to exploit the persistence of a pre-selected transient effect arising from the interaction of a previous pulse with the material, said laser pulses having a pulse width of less than about 10 picoseconds, and collectively having fluence above a threshold value for modification of said material.
The invention may also provide a method of laser material processing, comprising providing a material to be processed and applying laser pulses to a target zone on the material, the laser pulses having a time separation between individual laser pulses sufficiently long to permit hydrodynamic expansion of a plume and/or plasma so that a next subsequent laser pulse is not substantially reflected, scattered and/or absorbed by the plume and/or plasma, and the laser pulses having a time separation between laser pulses sufficiently short so that a thermal and/or other relaxation process (for example, mechanical, stresses, melt phases, metastable or long-lived states, transient species, shock waves, discoloration, deformation, absorption spectrum, fluorescence spectrum, chemical structure) in the target zone presents heated material or material alternated from the relaxed state to successive laser pulse(s).
The laser pulses may be applied at rates above 100 kHz, wherein thermal transport does not completely dissipate the heat deposited and/or transported in or near the processing volume by each laser pulse, or wherein other relaxation processes have not fully dissipated in or near the processing volume of each laser pulse. A region of warmed material is therefore preserved, and presented to each subsequent laser pulse.
This thermal component and other relaxing processes offer a new modality for controlling ultrafast-laser processing. By adjusting the pulse-to-pulse separation (inverse of repetition rate), the temperature rise, and the extent of the residually heated zone is controlled. In another embodiment, a subsequent laser pulse can be presented at a critical time in the evolution of material properties in or nearby the laser interaction zone to alter the subsequent laser interactions for a controlled change and/or improvement in the laser process. For material heating, subsequent laser interactions offer several advantages and opportunities that are not available for material processing at lower repetition rate, as for example, when the sample interaction has relaxed to close to the substrate temperature. An increased temperature dramatically alters the materials properties in a manner that can positively affect the ultrafast interaction, and control subsequent events such as shock development, defect formation, annealing, surface morphology, debris formation, plume evolution, material removal rates, and geometry of excisions. The combination of high-repetition rate with ultrafast laser pulses provides added control and new avenues in material processing that have not been described before.